I was recently doing some rewiring to install dedicated circuits for networked Computer equipment. I was looking for a certain breaker (should have been marked) to turn off approx 5 duplex receptacles which had 2 Computer/Monitor/UPS systems and 1 Adding Machine. After going through every breaker 3 times and following wiring (visible entire length) I found that it was already on it's own Breaker. We finally found that after the breaker for that circuit was turned off, it took 5 - 10 seconds for the voltage to go down! After unplugging the small UPS unit I immediately tested the prongs on the plug and found that somewhere around 110v was being backfed through the plug! and into the receptacle!
I would say it's against (Canadian) standards to have the energy available after it was disconnected. Obviously, this UPS is defective. Check the bi-national standard for ITE (UL1950/CSA 950) for the proper provision/s.
I agree that it is defective and have removed it from service. My reasons for asking were:
1. I was wondering if Backfeeding was a common result of some particular component failure within the UPS. If so, (it was an older unit) have newer units been equipped with some provision to prevent this from happening?
2. And should the UPS units within a facility be suspected and tested as part of a search for unexplained power quality fluctuations?
Boy, this is an old one! No, I haven't seen anything like that again. I was meaning to have someone look at it and think I still have the unit on a shelf somewhere. I know one person that could probably tell me why and will try to get it to him as I am still curious about it. I'll post back here if I can get an answer.